Problem

Predict. which of the following will be coloured …

Problem 17 Easy Difficulty

For $\mathrm{M}^{2+} / \mathrm{M}$ and $\mathrm{M}^{3 *} / \mathrm{M}^{2+}$ systems the $E^{\circ}$ values for some metals are as follows:
$\begin{array}{llll}\mathrm{Cr}^{2+} / \mathrm{Cr} & -0.9 \mathrm{~V} & \mathrm{Cr}^{3} / \mathrm{Cr}^{2+} & -0.4 \mathrm{~V}\end{array}$
$\mathrm{Mn}^{2+} / \mathrm{Mn} \quad-1.2 \mathrm{~V}$
$\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}+1.5 \mathrm{~V}$
Use this data to comment upon:
(i) the stability of $\mathrm{Fe}^{3+}$ in acid solution as compared to that of $\mathrm{Cr}^{3+}$ or $\mathrm{Mn}^{3+}$ and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

Answer

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Chemistry 102

NCERT Class 12 Part 1 - Chemistry

Chapter 8

The d-and f-Block Elements

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Video Transcript

okay for this particular problem way need to determine what the two reactions are? To identify the two reactions, we need to know what products are. It's not obvious what all of the products are for. The first reaction from the manganese metal reacts with nitric acid producing mn two plus and something else. You scour the half reactions in the back of the book. You could find 1/2 reaction where you know three produces and oh, and this ends up being the product. All know, although not explicit in the problem, so we could then right to half reactions that comprise this entire reaction. 1/2 reaction would be the No. Three minus an acidic solution, plus forage plus and three electrons goes toe noor water. The reduction potential for this process is 0.90 volts. Then, for the second half reaction, it would be making these two plus going to manganese solid. This has a reduction potential of negative 1.18 Now it's obvious in the balance reaction. I'm sorry in the actual chemical reaction that manganese is a reactant, not a product. Therefore, this chemical reaction this half reaction will be reversed. This will service the ANOTE, and that's what the reduction potential suggests. That has a lower reduction potential, so this reduction won't occur, but the first reduction will occur. So if this half reaction is reversed and we multiply, um, this half reaction by three in the first half, reaction right to we can then some these half reactions and get the overall balanced Redox reaction for this process. This is part pay of the first part. Have the for the first reaction, Then we to calculate he sell, we're going to do Catholic potential minus and a potential which was for these two voltages, which are found in the A table of production potential. She is going to be negative n f e and is the number of moles of electrons transferred in the balance. Chemical reaction, the balanced chemical reaction. You'll see that we made the number of electrons equal to six. So we've got Delta G is equal negative six multiplied by Faraday's constant applied by the cell potential that we just calculated. This thing gives us a value of negative 1.2 million jewels or negative one point to start a negative. 1200 something 39 kill a jewels for the second reaction involved in this process of determining the amount of manganese. And still we have the priming grenade being formed from the pariah state. So we need to find 2/2 reactions that contain permanganate and right and the 2/2 reactions that are found in the table of reduction potentials are this 1st 1 here where permit from Inga Nate goes to and two plus and it is mentioned from that. Um no. Well, okay, it is mn two plus that is reacting with the Brian. So the fact that we have them in two plus here and permanganate here is good. It is likely, then, that this half reaction will be switched so the MN two plus can become from Inga date. Then it mentions that the pariah state when read when reacting, um, it doesn't say what it produces. Eso that's unfortunate. But there is only 1/2 reaction that has pariah date in it, making just the I a date. So we can assume that this is the other half reaction that comprises the overall reaction, and it has a reduction potential of 1.60 So in order to get a chemical reaction, where and then two plus reacts with pariah state for mean permanganate, then we need to change the direction of this first reaction and leave the direction of the second reaction, as is, which is good because it has the greater reduction potential. So it will be the one that wants to be reduced war and will be reduced, and this half reaction will then serve as the so. If we do that, we reverse the direction of this first half reaction. Multiply the second half reaction by five in the first half, reaction by two so that we did 10 electrons that can be canceled. Then we will get this overall balanced Redox reaction, making sure that we cancel any waters that air comment herbal sides and any hydrogen ions that are common table sides. So we end up with a net three waters on the right hand side and the Nets 609. I'm sorry, through hunters on the left hand side and in that six waters on the right hand side, then the cell potential is simply going to be the Catholic potential, minus the AM a potential giving us a 0.9 volts. And then Delta G will be negative and f d and is 10 as that was the number of electrons that were canceled as we were. Dr. Reaction, Remember, cell voltage is 0.9 giving us negative. 86,900 jewels were about negative. 86.9 killer jewels.

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